Post heating of sputtered metal oxide films

ABSTRACT

This invention relates to a novel method of producing electroconductive metal oxide films by cathodic sputtering. It especially pertains to a method of increasing the electroconductivity of sputtered metal oxide films by mildly reducing said films. The reduction step follows sputtering and generally involves heating the metal oxide film in a nonoxidizing or reducing atmosphere for a sufficient period of time to reduce the oxygen content of said film by a minor amount but without substantially reducing the oxide film to metallic form.

United States Patent Gillery et al. [451 Apr. 11, 1972 541 POST HEATING0F SPUTTERED 3,336,661 8/1967 Polinsky ..317/235 METAL OXIDE FILMS3,108,019 10/1963 Davis 2,825,687 3 1958 P [72] Inventors: Frank H.Gillery, All1son Park; Jean P. 2,597 562 41952 :232:23

Press, Evans 3,420,706 l/l969 K110 ..117/62 Z STE i giz' FOREIGN PATENTSOR APPLICATIONS 1 Juy 566,773 12/1958 Canada ..204/192 [21] Appl.No.:56,117

Primary Examiner-John H. Mack Related Apphcam Data AssistantExaminerSidney S. Kanter [63] Continuation of Ser. N0. 709,135, Feb. 28,1968, Atmmey(3hish9lm8981191199r abandoned.

[57] ABSTRACT [52] U.S.Cl ..204/192 This invention relates to a novelmethod of reducing elec {51] in C1 C23C 15/00 P [58] d 204/192troconductive metal oxide films by cathodic sputtering. lt 1e 0 respecially pertains to a method of increasing the electrocom ductivityof sputtered metal oxide films by mildly reducing [56] References andsaid films. The reduction step follows sputtering and generally UNITEDSTATES PATENTS involves heating the metal oxide film in a non-oxidizingor reduclng atmosphere for a sufficient period of time to reduce3,506,556 4/1970 Glllery et al. ..204/192 the Oxygen content f Said m bya minor amount but without 3,438,885 4/1969 Lewls et a] "204/192substantially reducing the oxide film to metallic form. 3,386,906 6/1969Bronnes ..204/ 192 3,370,978 2/1968 Pollack et a1. ..117/217 10 Claims,1 Drawing Figure PATENTEDAPR 1 I I972 caravan 4- VIIIIIIIIIIIA mam 4 4,AND/OB REACT IVE GASES COOLING MEDIUM INVENTORY f-H- G/lLEfiY 4 J R. Bet$514 U fima POST HEATING F SPUTTERED METAL OXIDE FILMS This applicationis a continuation of application Ser. No. 709135, filed Feb. 28, 1968,now abandoned.

BACKGROUND OF TECHNOLOGY The deposition of metals and metal oxide filmsby cathodic sputtering is a well-known process. U.S. Pat. No. 3,242,006discloses a process for the preparation of tantalum nitride by cathodicsputtering. In cathodic sputtering processes, the applied voltageenergizes gaseous ions in a vacuum chamber and causes such ions tostrike a cathode, thereby displacing a metal particle. The metalparticle migrates to the substrate to be coated, said substratefrequently being an anode.

In cathodic sputtering processes, a vacuum of greatly reduced pressureis necessary to provide the proper conditions for a glow discharge tooccur between the cathode and the anode. A glow discharge energizesgaseous ions present between the cathode and the anode.

An alternate technique for depositing metal oxide films on a substratecomprises contacting a heated substrate with a metal salt ororganometallic compound which pyrolyzes at the temperature of thesubstrate to form thereon an adherent metal oxide film. One disadvantageof pyrolyzation processes resides in the elevated temperatures necessaryfor depositing a suitable metal oxide film. When glass is utilized asthe substrate, for example, the necessary pyrolyzation temperaturesapproach the softening point of the glass, thereby causing undesirableoptical distortion in the glass. Cathodic sputtering processes, however,do not require the substrate to be heated to elevated temperatures. Suchprocesses are especially amenable, therefore, for producing transparentmetal oxide films on high quality optical glass substrates.

Metal oxide films, regardless of the method of application, have onedisadvantage: low conductivity. Transparent tin oxide films, forexample, frequently have resistances of over 2,000 ohms/square for a6,000 A thick film (specific resisitivity of about 0.12 ohms cm.), whilepure metallic films such as tin or copper may have resistances of lessthan 1 ohm/square for similar film thicknesses. Thin metal oxide films,however, have much better adhesion, durability, and light transmissionthan the pure metal films. One objective of research in this area hasbeen the development of an adherent metal oxide film which has aconductivity approaching that of the metal films.

One successful approach towards improving the conductivity of a metaloxide film has involved doping with another metal, generally one of ahigher valence. The doping metal apparently takes a position in thelattice of the metal oxide film and provides a greater electron density.

The doping of vacuum-deposited tin oxide films with antimony hasproduced transparent tin oxide films having resistances less than 1,000ohms/square. Such developments have been reported by Holland in histext, Vacuum Deposition of Thin Films," Chapman and Hall, Ltd., London(1963) at page 497. Indium oxide films, for example, doped with tin havebeen produced by sputtering to have resistances of as low as aboutohms/square at a thickness of about 6,000 A.

Another technique of producing conductive oxide films of tin or indiumis disclosed in U.S. Pat. No. 2,769,778 issued to Preston. Prestonsputtered tin or indium in a deficiency of oxygen to form asubstantially metallic film which was later converted to an oxide filmby heating under oxidizing conditions. Film resistances of 500 to 1,000ohms/square were achieved at 500 microns film thicknesses.

INVENTION It has now been discovered that the conductivity of pure metaloxide and doped metal oxide films produced in the presence of oxygen bycathodic sputtering or other means can be increased by subjecting suchfilms to reduction. The reduction is conducted after a film of desiredthickness is deposited. The effect is novel inasmuch as the metal tooxygen ratio of the film is altered only very slightly; in fact, thechange is practically undetectable. A substantial decrease in the oxygencontent of a transparent film is readily detected inasmuch as the lighttransmission of the film substantially decreases.

The reduction process of this invention is conveniently obtained byheating, at a minimum temperature of about 200 C. and, preferably, atemperature of about 240 to 340 C. The duration of heat treatmentdepends on the time taken for the films to come into equilibrium withthe surrounding atmosphere, in practical cases, about two hours or less.

The gas in contact with the film during heating is an important part ofthe process. The heat treatment may be conducted under vacuum, atatmospheric pressure or at elevated pressures. It has been found thatheating in oxygen-containing atmospheres increases the conductivity ofmetal oxide films. At temperatures of less than about 300 C., the oxygencontent of air may be insufficient to oxidize further metal oxide filmsproduced by sputtering in an oxygen containing atmosphere. However, ithas been found that a significant increase in conductivity is obtainedif the atmosphere is inert, that is, contains no oxygen, or, moresignificantly, if hydrogen or some other reducing agent is present.

Over reduction of the oxide films should be avoided since this resultsin a dark film having reduced light transmission. Over reduction may becaused by heating at extremely high temperatures or heating for anextended duration. Control of the operation may readily be establishedfor a particular type of film by initially heating stepwise, checkingconductance after each heat treatment, and continuing heating until adesired or optimum conductance and light transmission is achieved.Generally, observance of the temperature limits set forth herein willpreclude overheating, especially if the duration of heating is shortenedas the temperature is increased.

Besides the gain in conductivity of the film due to reduction, some ofthe increased conductivity is also due to annealing and densification ofthe film by the heat. This is commonly experienced in many metal filmsand may partially explain the increase in conductivity obtained byheating metal oxide films in the presence of an oxygen containingatmosphere. The two effects can be distinguished, because the reductioneffect is partly reversible, that is, this part of the conductivity gaincan be lost by reheating in a more oxidizing atmosphere, whereas thedensification effect is irreversible. In theory, the reduced state ofthe film could be obtained by sputtering in an atmosphere containingonly argon with less than the usual concentration of oxygen. In practiceit is very difficult to control all the conditions required in thevacuum system to obtain this result. If the oxygen concentration is toolow, dark films are obtained, indicating the presence of a lower oxideof the metal or sometimes the metal itself.

The invention is also applicable to metal oxide films doped with a metalhaving a higher valence state. For example, indium oxide films are dopedwith tin oxide while tin oxide films are doped with antimony oxide. Thedoped metal oxide films have considerably lower resistances than thepure metal oxide films. The present invention is especially usefulinasmuch as it presents a technique for making further improvements inthe conductivity of doped metal oxide films.

This invention is also particularly applicable to transparent metaloxide films. Additional conductance may be obtained in opaque metaloxide films by increasing the thickness of the film. However, when ahighly transparent metal oxide film is desired, the specificconductivity of the film itself must be increased in order to achieveany increase in film conductance without sacrifice of transmission.Transparent metal oxide films, that is, those having a thickness of lessthan about 6,000 angstroms have been produced according to the teachingof this invention with resistances as low as about 15 ohms/square orless. When hydrogen is present during the post-heating step, theultimate resistance for a doped metal oxide film is about 10 ohms/squareor less. For example, indium oxide films doped with tin oxide have beenproduced according to the teaching of this invention with a filmresistance of about 10 ohms/square for a film thickness of about 6,000angstroms.

For a better understanding of the invention, reference is now made tothe figure which depicts a typical sputtering apparatus.

The sputtering apparatus is composed of a vacuum chamber 2 whichcontains a cathode 1 which is preferably constructed of the metal whoseoxide is to be deposited on the substrate 3. The substrate 3 issupported by a substrate support 4 which may be heated or cooled toimprove the properties of the metal oxide. The substrate support 4 canbe grounded to form an anode. The cathode 1 is connected to ahigh-voltage supply 7 and rectifier 9 which create a high-potentialdifferential between the cathode l and the substrate support 4 (anode).The high-potential differential provides the glow-discharge necessary tocause deposition of the metallic oxide from the cathode onto thesubstrate. The vacuum is obtained by a vacuum pump which exhausts thevacuum chamber to a pressure of about 20 millitorr or lower. Higherpressures may be utilized, for example, up to 40 millitorr and above;however, the applied voltage may require adjusting to achieve a suitableglow-discharge.

For the purposes of this invention, the vacuum chamber is equipped withan inlet port 6 for introducing inert and/or reactive gases into thevacuum chamber. After the proper vacuum is obtained, preferably apressure of less than about 10 torr, the required atmosphere forsputtering is obtained by introduction of a small amount of the desiredgas which frequently comprises at least a small quantity of an inertgas.

A typical procedure for depositing a metallic oxide film utilizing theabove-described apparatus involves the application of about 2,500 voltsto the cathode after the system has been evacuated to a pressure ofabout 20 millitorr. The voltage applied to the system is that necessaryto obtain a suitable glow-discharge and, therefore, will vary withpressure, cathode to substrate distance, gas composition, and the like.A cathode of the dimensions of 12.5 centimeters by 12.5 centimeters ispositioned 25 millimeters above a glass sample which is a 10 centimetersquare. The indium cathode is cooled by cooling means 8 which is aminiature heat exchanger cooled by introduction ofa cooling medium suchas a cool gas or a cool liquid.

The atmosphere in the vacuum chamber may typically contain about 40 toper cent oxygen and about 60 to 85 per cent argon, although widevariations of atmosphere composition are useful. The pressure of thevacuum chamber after introduction of the appropriate gases should be inthe range of about millitorr. The substrate temperature should bemaintained in the range of about 300 C., although temperatures as low asroom temperature may be used. Operation under these conditions resultsin formation of a transparent conductive film having a resistance ofabout 280 to 700 ohms/square after about 60 minutes of operation. A widerange of operating conditions is feasible, attended by a wide variationin the properties of deposited films.

The invention described herein is especially useful in that it isespecially adaptable for producing transparent metal oxide coatings ofvarying conductivities. Films of various conductivities may be achievedby varying the duration of the postheating cycle. Although it is knownthat lowering the oxygen concentration in the sputtering atmosphereincreases conductivity, an increase accomplished by a reduction of theamount of oxygen present may detrimentally affect the light transmissionof the films. The resulting film would have properties approaching thatof a pure metallic film which has considerably less light transmissionand less adhesion to substrates, especially glass, than a metal oxidefilm.

Various types of metal oxide films may be deposited by the technique ofthis invention. Especially good films of oxides of a metal having anatomic number between 48 and 51, for example, tin oxide, indium oxide,and cadmium oxide, may be deposited by sputtering in anoxygen-containing atmosphere and subsequently heating to a temperatureabove about 240 C. in a non-oxidizing atmosphere. The conductivity ofthe resulting metal oxide films is about an order of magnitude greaterthan that of the metal oxide films not subjected to a post-heattreatment. For example, as indicated above, transparent sputtered indiumoxide films doped with tin generally have a mean resistance of about 50ohms/square. However, tin doped indium oxide films of similarthicknesses have been produced by the teaching of this invention withresistances of about 20 ohms/square; a significant increase inconductivity (conductivity being the inverse of resistivity) of thefilm. Through optimization of the operating conditions, films of usefultransmission having resistances as low as 15 ohms/square or lower areproducible in reasonable industrial times.

In using the term metal oxide it is intended to refer to. the highervalent and lower valent oxides of a metal which may exist in more thanone valence state. Generally, it is the higher valent metal oxide whichis present in the deposited film. However, substantial quantities of thelower valent oxide may be present. For example, tin oxide films arebelieved to consist primarily of stannic oxide, although minorquantities of stannous oxide may be present, especially when adeficiency of oxygen exists in the sputtering atmosphere.

The operating conditions for the sputtering process of this inventionare similar to those of prior art sputtering processes. A minimumvoltage of about 50 volts is required to achieve a glow discharge whilea minimum voltage of about 100 volts is required to achieve a build upof metal oxide deposits within a reasonable time. A preferred operatingvoltage is above about 1,000 volts and, for commercial operations, avoltage of over 1,500 volts is recommended.

The distance maintained between metal cathode and substrate varies withthe cathode area, power utilized, gas pressure, and the like. Usualdistances are 2] to 35 mm, although greater and lesser distances may beutilized.

The operating pressure is generally about 20 millitorr althoughpressures as low as 5 millitorr are useful. Also, pressures of the orderof 100 millitorr or higher may be successfully utilized. The process isoperable at lower pressures in the presence of a magnetic field. Theutilization of higher pressures involves increased collisions betweenthe migrating particles and the gas atoms of the atmosphere, therebydiminishing the rate of deposition.

The content of the sputtering atmosphere may be varied considerably. Aninert gas, such as argon, nitrogen, or the like, may or may not bepresent. If an inert gas is utilized, it may be present inconcentrations of less than 1 per cent by weight to about 94 per cent byweight of the total weight of gases present. It is generally preferredto have an inert gas present, preferably in concentrations of about 5per cent to about 87 per cent by weight of the gases present. Theheavier inert gas ions enhance the sputtering rate. The oxygenconcentration may vary from about 90 per cent or more to about 5 percent or less. As noted above, films deposited in an atmosphere of highoxygen concentration tend to have high resistivity (low conductivity)while films deposited in an atmosphere having a low oxygen concentrationtend to have greater conductivity but less light transmission. For mostpurposes, a preferred oxygen concentration for sputtering of metal oxidefilms is between about 10 per cent and 60 per cent by weight of thetotal gases present.

As previously indicated, the cathode comprises a metal having an atomicnumber between 48 and 51, that is, cadmium, indium, tin, and antimony.These metals should be substantially pure when used for sputtering,although particular impurities may enhance the sputtering rate. Forexample, a metal of higher atomic weight in minor quantities, that is,up to about 20 per cent by weight, and preferably of less than 15 percent by weight of the total cathode weight, enhances the sputteringrate. Tin has a faster sputtering rate than indium and, therefore,increases the overall sputtering rate. Antimony acts as a dopant fortin.

The substrate temperature may be controlled by cooling, if desired. Bycooling the substrate, additional power may be applied and thesputtering rate thereby increased.

The following examples illustrate specific embodiments of theaboveidescribed invention.

EXAMPLE I Transparent indium oxide films were prepared by sputtering inan oxygen-argon atmosphere at a voltage of 2,500 volts and current of750 MA for a period of 60 minutes. The cathode to substrate distance was27 millimeters. The sputtering atmosphere composition and pressure wasvaried as well as the anode (substrate) temperature.

The indium cathode contained various quantities of tin as a dopant.Following sputtering, each film was post heated in hydrogen for a periodof two hours at 245 C. The following table presents the results of thisexperiment.

about 11 ohms/square without any detectable change in luminoustransmission.

Sample D was untreated and retained as a control.

The post heating in hydrogen was conducted by disposing a sample to betreated in a horizontal Vycor tube. The Vycor tube was part of a tubefurnace wherein the open ends of the tube extended from either side ofthe fumace. Hydrogen was introduced at one end of the Vycor tube andburned at the other end. A thermometer placed in the Vycor tube measuredthe sample temperature.

The term ohms per square has been used hereinabove to describe theconductivity of the films formed by the novel process of this invention.Although specific resistivity is 15 usually utilized to describe orcompare the conductivity of TABLE I Percent Percent Oper- Sn in 02 inating Anode Fihn color Resls- After Sn-In O2A pressure temp. by tance,heatln Sample alloy mixture torr C. reflectance ohms/sq. resistance 750.045 340 6thgreen 60 24 95 0.046 397 6thgreen.. 19 35 0.050 3185thgreen 50 30 75 0.050 300 5thred 82 48 75 0.075 396 7thgreen 22EXAMPLE [I Another group of glass plates were coated to a thickness ofapproximately 5,000 A from a cathode of 100 per cent indium bysputtering for one hour at a voltage of 2,100 volts and current of 750MA. The sputtering atmosphere was 25 per cent by volume oxygen and 75per cent by volume argon. The resistance of each film was about 370ohms/square.

Three of these samples weresubjected to post heating at a temperature of300 C. for 60 minutes. Each of the samples was post heated in adifferent atmosphere. The results were as follows:

Resistance Post Heating (ohms/square) Atmosphere After HeatingH,(5%)+N,(95%) 24 Natural Gas 24 Hydrogen 13 The resistance in eachinstance was substantially reduced from the original resistance of 5 70ohms/square, exemplifying the advantageous nature of limited reductionafter sputtering. The luminous transmission of each sample-wassubstantially unchanged by this post heating.

EXAMPLE [II An indium oxide film was sputtered onto a 4 X 4 X 1/8 inchglass substrate from an indium cathode containing 1.4 per cent by weighttin under the following conditions:

Power 2,350 volts at 750 MA Cathode to sub- 27 millimeters stratedistance Atmosphere 75 per cent 0 and 25 per cent A Pressure 43millitorr Sputtering time One hour reduced from an average of about 115ohms/square to 75 materials, it is inappropriate for describing theconductivity of very thin films because of the difficulty of measuringthe thickness of the film.

Specific resistance is the resistance between opposite faces of a cubiccentimeter of material and is expressed by the equation where p is thespecific resistance, R is the resistance of the conductor, A is thecross-sectional area of the conductor, and L is the length of theconductor. For a thin film, this expres- 40 sion becomes 45 wherein Wand L are the surface dimensions and t is the film thickness. For asquare area of surface, Wand L are equal and p R X t or R (resistancefor a square area of surface) p/t. Thus, the conductivities of varioustypes of films having approximately equivalent thickness may be directlycompared by comparing resistance per square.

, The thickness of a thin film may be determined by the interferencecolor shown in reflected light, provided the index of refraction isknown. For stannic oxide films, a second-order red color indicates athickness of about 230 millimicrons while a second-order blue colorindicates a thickness of about 100 millimicrons. As the thickness of thefilm increases, its apparent color changes and the order or successionof the colors with increasing thickness is analogous to that of thewellknown Newton rings described in The Theory of Optics by Paul Drude,Dover Publications, Inc., New York, at page 136 et seq.

Although the instant invention has been described with reference tofilms produced by cathodic sputtering, it should be recognized that itis applicable to films produced by other vacuum deposition techniques,e.g., by thermal evaporation.

Although specific examples of the instant invention have been set forthhereinabove, it is not intended that the invention be limited solelythereto, but should include all the variations and modifications fallingwithin the scope of the appended claims.

What is claimed is:

1. A method of producing a transparent, electroconductive metal oxidefilm exhibiting a low electrical resistance on a substrate comprisingcathode sputtering said film on said substrate from a metal cathodesource containing one or more metals having an atomic number between 48and 51 in an oxygen-containing atmosphere consisting essentially ofabout to about 90 per cent by weight of oxygen in an oxygen-inert gasmixture at a maximum temperature of about 400 C. until a transparentmetal oxide film of the desired thickness is obtained, then post heatingsaid film to a temperature between about 200 C. and about 340 C. in anon-oxidizing atmosphere for a period of time sufiicient to lower theelectrical resistance of said metal oxide film and discontinuing saidpost heating before the transparency of said film is substantiallyimaired.

p 2. The method of claim 1 wherein the post heating is conducted invacuo.

3. The method of claim 1 wherein the post heating is conducted in areducing atmosphere.

4. The method of claim 1 wherein the post heating is conducted in ahydrogen-contain%mosphere.

5. The method of claim 1 wherein the substrate is glass.

6. The method of claim 1 wherein post heating is conducted at atemperature of about 240 C. to about 340 C.

7. The method of claim 1 wherein the metal cathode contains indium.

8. The method of claim 7 wherein the post heating is at a temperature ofabout 240 C. to about 340 C. for less than about two hours.

9. The method of claim 1 wherein the metal cathode contains a greaterquantity of indium that that of tin.

10. The method of claim 9 wherein the metal cathode contains a maximumof 20 percent by weight of tin and the balance indium.

2. The method of claim 1 wherein the post heating is conducted in vacuo.3. The method of claim 1 wherein the post heating is conducted in areducing atmosphere.
 4. The method of claim 1 wherein the post heatingis conducted in a hydrogen-containing atmosphere.
 5. The method of claim1 wherein the substrate is glass.
 6. The method of claim 1 wherein postheating is conducted at a temperature of about 240* C. to about 340* C.7. The method of claim 1 wherein the metal cathode contains indium. 8.The method of claim 7 wherein the post heating is at a temperature ofabout 240* C. to about 340* C. for less than about two hours.
 9. Themethod of claim 1 wherein the metal cathode contains a greater quantityof indium that that of tin.
 10. The method of claim 9 wherein the metalcathode contains a maximum of 20 percent by weight of tin and thebalance indium.